The first extended preview of The Flash and Supergirl's musical crossover is a Glee reunion (with super powers!).

Melissa Benoist (Supergirl) and Grant Gustin (The Flash) get to sing and dance again with their old Glee co-star Darren Criss - when he's not trying to kill them, of course!

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The CW's extended preview confirms that, yes, the two-night musical crossover does take place in some sort of dream world - where Supergirl and The Flash have to "follow the script" of the evil Music Meister's all-singing, all-dancing thriller if they want to stay alive in real life.

It's a delightful reversal of fortunes, since it was Grant Gustin's sadistic Sebastian Smythe who tortured Criss's Blaine Anderson when they both ended up on the Warblers choir in Glee.

Somehow Barry's surrogate dad Joe West (Jesse L Martin), the sinister Malcolm Merlyn (John Barrowman) and even one-half of Firestorm, Martin Stein (Victor Garber), end up in this noir-ish musical nightmare too - presumably all picked because the actors who play them have extensive musical theatre backgrounds.

Victor Garber is the Tony Award-nominated star of such classic Broadway productions as Sweeney Todd,Damn Yankees and You Never Can Tell, while Jesse L Martin shot to fame in the original cast of Rent.

John Barrowman had his own West End success in the revival of Anything Goes and has released covers albums of Broadway standards. So yeah, the singing in these episodes is going to be on-point.